This invention relates to transmission over metallic wire.
In a Plain Old Telephone System (POTS) environment, a twisted copper pair is typically employed to a bandwidth of approximately 4 KHz, but recently several techniques have been created to increase the available bandwidth. These techniques belong to the group of Digital Subscriber Loop modulation techniques, commonly abbreviated as DSL, HDSL, SDSL, ADSL, etc. (xDSL, as a group).
ADSL (Asymmetric Digital Subscriber Line) is among the best known xDSL techniques. Pursuant to a "template" recommended by BELCORE, a low frequency band from 0 to 40 KHz is assigned to a duplex channel that can be used for POTS or ISDN service. The remainder of the allowed bandwidth, from 40 KHz to 1 MHz, is assigned to a unidirectional channel for wideband applications. The original ADSL embodiment utilized a modulation technique developed at Bell Laboratories in the 1960's which employs an array of individual low-rate modulated carriers to convey high-rate information (rather than one modulated carrier with wide sidebands). The technique is computationally intensive so it was basically unused until the advent of powerful signal processing ICs which allowed the use of Fast Fourier Transform and Inverse Fast Fourier Transform signal processors for reception and transmission, respectively. The resulting technique was embodied by ANSI in T1.413-1995 and is known as Discrete Multitone Modulation, or DMT.
The nominally achievable data rate over the wide bandwidth channel ranges from 1.5 to 6.1 Mbps. However in practice, over a multi-pair cable, the rate is substantially smaller. The actual achievable data rates depend on a number of factors, including the length of the copper line, its wire gauge, presence of bridged taps, varying humidity, changes in characteristics over time, and interferences, etc. The interferences come from like signals that are flowing through adjacent wires of a cable, from external sources that are far-removed, such as AM radio transmissions, and from near-by isochronous and asynchronous traffic. What is known is that the XDSL techniques are poorly adapted to deal with such interferences and, therefore, it remains to be seen what actual data rate is achievable in an environment of a cable with many wire pairs that carry isochronous and asynchronous traffic.